Abstract: | In Elodea densa leaves light strongly stimulates electrogenic,K +-dependent, vanadate- and erythrosin B-sensitive H+ extrusionand hyperpolarizes the transmembrane electrical potential. Theseeffects of light are suppressed by treatment with DCMU, an inhibitorof photosynthesis, which has no effect on H+ extrusion in thedark. Light-induced H+ extrusion requires the presence of K+in the medium and is associated with increased K+ uptake andalkalinization of the cell sap. Light-induced H+ extrusion increaseswith increased CO2 concentration. At constant CO2 concentration(104 parts 106) the rate of H+ extrusion is stronglyenhanced by an increased light intensity up to 30 W m2.Different wavelengths, between 400 and 730 nm, induce a significantstimulation of both proton secretion and transmembrane potentialhyperpolarization. The stimulating effects of light on H+ extrusion, K+ uptakeand cell sap pH are very similar to those induced in the darkby fusicoccin, a toxin known to stimulate strongly ATP-driven,vanadate- and erythrosin B-sensitive H+ transport. In the light,the effects of fusicoccin are only partially additive to thoseof light, thus suggesting that the two factors influence thesame system. The identification of this system with the plasmamembrane H+-ATPase is indicated by the observed inhibition ofthe effects of either light or fusicoccin by the H+-ATPase inhibitorsvanadate and erythrosin B. These data indicate that the activation of electrogenic H+ extrusionand of K+ uptake by light is mediated by some products of photosynthesis.The mechanism and the possible physiological implications ofthis phenomenon are discussed. Key words: Photosynthesis, H+ pump, K+ uptake, Elodea densa |